Of biology's eleven themes, evolution is one of the major unifying themes. The main idea of evolution is that all life on Earth shares a common ancestor. It is descent with modification as Charles Darwin said. As organisms continue to reproduce, certain traits will be passed down and some will not. When this happens, organisms are better equipped for life and to reproduce than their predecessors. There are four different mechanisms that lead to evolutionary change: gene flow, genetic drift, mutation and natural selection.
Gene flow is the transfer of alleles in and out of a gene pool. In other words, the migration of an organism into another population or area and reproduces will cause allele frequencies to change. It tends to reduce the differences between populations as organisms intermix with each other. As they continue to reproduce, gene flow can change neighboring populations to bind together to make one population with a common gene pool.
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Allele frequencies can fluctuate unpredictably from generation to generation because the smaller the population or sample, the greater the chance of the alleles deviated from the predicted result. Genetic drift is the random change in allele frequency. What can increase the chances of genetic drift are two situations known as the bottleneck effect and the founder effect. When there is a sudden change in the environment it can drastically reduce the size of a population. When a fire or flood occurs, the survivors do not have the gene pool of the original population. The probability that certain alleles be over or underrepresented will occur. As for the founder effect, when a small group gets separated from the original population and make a new population, their gene pool will not be the same as the original population. The founder effect occurs when there is colonization in a new location. This new distinct gene pool will have different allele frequencies then those of the original population.
Changes in the nucleotide sequence of DNA are called mutations. It is not possible to tell how a mutation will change a DNA and what will happen. One single change in a gene is a point mutation. Point mutations are normally harmless but can have a significant impact on phenotype as in sickle-cell disease. The almost certain to be harmful mutations are the chromosomal mutations that can delete, disrupt, or rearrange many loci at once. Mutation rates in animals and plants are about one in every 100,000 genes per generation. But in viruses with short generation spans, mutations can generate rapidly.
The primary mechanism of adaptive evolution is natural selection. Genetic variations that are heritable are the raw material for natural selection. Natural selection increases the frequencies of certain genotypes, fitting organism to their environment. Fitness is the input an individual makes to the gene pool of its offspring. For example, a moth having more offspring due to its concealing body colors or a barnacles ability to produce more eggs due to its ability at collecting more food. Flowers with more attractive colors, shapes or fragrance will be pollinated more than those that do not hold the same qualities. It will slowly but surely the populations of the less attractive flowers will decline since pollinators will not go to them as often. Over a few generations they could even disappear.
There are three different selections that can alter the frequency distribution of heritable traits. Directional selection which can happen when a population's environment changes or when an individual moves to a new environment different from their former one. Disruptive selection happens when conditions favor organisms with phenotypic ranges than the organisms with intermediate phenotypes. Stabilizing selection acts against the severe phenotypes and prefers intermediate phenotypes.
Natural selection also has constraints since it cannot create perfect organisms. Each organism has a legacy of descent with modification from its ancestors. New structures are not made but improvements to the existing structures are made to fit the current generation's environment. In order to be an expert in certain areas a compromise must be made. In an individual is an expert swimmer, it is very likely that it cannot survive on land. Natural selection favors the fittest phenotypes but cannot make new ones on demand. At times, these phenotypes will not be the ideal traits.
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Scientists know that gene flow, genetic drift, mutation and natural selection are all mechanisms for change and use bacterial resistance to antibodies, comparative biochemistry and fossil records as evidence of evolution. Bacteria become resistant to antibodies by either mutation or horizontal gene transfer-when certain bases are swapped in DNA by bacteria. The past diseases or troubles that hurt individuals may leave genes with mutations that leave them to be resistant to antibodies. They then pass these traits on to their offspring. Fossils are the remains of animals preserved in sedimentary rock. Evolution can be seen in by the similar items that are present day animals that have evolved from those in the fossils.
Comparative biochemistry can be applied to all types of organisms. The human amino acid sequence is similar to that of chimps' but not to that of a kangaroo. This speculates that humans and chimps come from a common ancestor. Common ancestors that reproduce and had their offspring go through one of the four mechanisms of evolution to conform to their environment. Comparing organisms on a microscopic level show that some organisms come from common ancestors which prove that we have evolved into more capable beings.
Evolution is the frequency of genes in a population from a generation to the next and the descent of different organisms that come from a common ancestor. As life goes on, more and more changes are being made unbeknownst to us. It is amazing to think of the future organisms that will develop from present day organisms.